class_docstr: A stream operator that counts the number of non-missing values in the
  rolling window.
code: def forward(self):
      rolling = self.inputs[0]
      history = rolling.value
      return self.func(history)
  
imports: from typing import List, Callable
  import numpy
  from tensortrade.feed.core.base import Stream
  from tensortrade.feed.api.float import Float
label_desc: Use image node_id 2 for calling the RollingCount obj's underlying member method code with example usage: obj.forward() and returns: self